Method of selecting a reference picture

ABSTRACT

In one embodiment, the method includes obtaining a reference picture list including multiple reference pictures, and obtaining a field reference picture index associated with the current field macroblock. A frame reference picture index is determined based on the field reference picture index divided by 2, and a field reference picture is selected based on the frame reference picture index and the field reference picture index.

This is a continuation of, and claims priority under 35 U.S.C. §120 to,U.S. application Ser. No. 10/541,540, filed May 19, 2006, which is theU.S. National Stage of International Application No. PCT/KR/2004/000445,filed Mar. 3, 2004, the entire contents of all of which are herebyincorporated by reference in its entirety. This application also claimspriority from Korean Patent Application No. 10-2003-0013198 filed onMar. 3, 2003, in the Korean Intellectual Property Office under 35 U.S.C.§119. The entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to coding and encoding a moving picture.

BACKGROUND ART

Moving picture coding systems compensate for motion using motion vectorinformation. In the case of moving picture coding system that usesmultiple reference pictures, reference picture index information as wellas motion vector information is required for motion compensation. Thereference picture index is a value that is used to distinguish multiplereference pictures from each other. A coder transfers the referencepicture index to a decoder. The decoder selects the reference picture asindicated by the reference picture index, and performs motioncompensation using the selected reference picture.

Generally, scanning methods for images are classified into eitherprogressive scanning or interlaced scanning. In the case of theprogressive scanning, an image of one frame consists of data that aresampled at the same time. In case of the interlaced scanning, an imageof one frame consists of data that are sampled at different times, andthe samples are alternated line by line.

The interlaced image of a frame is usually divided into two fieldscalled a top field and a bottom field. In the interlaced scanning, oneframe is divided into two field images. In this specification, an imageis treated based on a unit of a picture. A frame or a field may bereferred to as a picture.

Three coding methods for an interlaced moving picture are proposed. Afirst coding method is a field picture coding method which performs acoding process, considering each field as an independent picture at apicture or slice level. A second coding method is the frame picturecoding method which performs a coding process after combining two fieldsinto one frame at a picture or slice level. A third coding method is aframe picture coding method with frame/field macroblock, which performsa coding process by combining two fields into one frame and selecting aframe mode or a field mode at a macroblock level.

In the third coding method, two vertically adjacent macroblocks arebound into a pair and the coding is performed in unit of a macroblockpair. In FIG. 1, numbers assigned in the macroblock pairs indicatemacroblock addresses that are used to distinguish the macroblocks fromeach other.

Here, a frame macroblock is a macroblock which is coded at upper andlower macroblocks of the macroblock pair in units of a frame. In otherwords, each macroblock is coded in units of a frame after two fields arecombined into one frame.

Meanwhile, a field macroblock is a macroblock that is coded at upper andlower macroblocks of macroblock pairs in units of a field. Themacroblock pair is divided into top and bottom field components and eachfield is coded independently. At this time, the macroblock pairs arerearranged so that the upper and lower macroblocks may respectivelycontain only a top field component and only a bottom field component.The upper macroblock is called a top field macroblock and the lowermacroblock is called a bottom field macroblock.

A reference buffer for storing multiple reference pictures is configuredin units of a frame. In the frame picture coding, all reference picturesare considered as units of a frame in which two fields are combined intoone frame, and one picture among the reference frame pictures is usedfor motion compensation. Accordingly, values are allocated to thereference picture indexes in units of a frame.

The reference picture index for a P frame is obtained by sorting all thereference frames in an order reverse to a coding order and thensequentially allocating an index, which is increased by one, to thesorted reference frames. The reference picture index for a B frame isclassified into a list 0 and a list 1, and is determined based on adisplay order of the reference frame.

First, in case of the reference frame list 0, indexes are allocated in areverse order to the reference frames whose display order lags behindthe B frame, and then, the remaining indexes are allocated in thedisplay order to the reference frames whose display order leads the Bframe. In case of the reference frame list 1, in contrast to thereference frame list 0, indexes are allocated in the display order tothe reference frames whose display order leads the B frame, and then,the remaining indexes are allocated in a reverse order to the referenceframe whose display order lags behind the B frame. FIG. 2 shows areference picture index for a P frame when a size of the referencebuffer is 5, and FIG. 3 shows the reference picture index lists 0 and 1for a B frame.

In the field picture coding, the fields of all reference pictures areconsidered as independent pictures and one of the reference fieldpictures is used for motion compensation. Accordingly, values areallocated to the reference picture indexes in units of a field. At thistime, the reference fields are combined in the unit of a frame, andthen, the reference picture indexes in each field of the P frame aresorted in an order reverse to a coding order of the frame. Then, indexvalues that are increased by one are alternately allocated in an orderthat starts from a reference field having parity equal to a currentpicture to a reference field having parity different from the currentpicture, while visiting the sorted reference frames in sequence.

In addition, in the case of the reference picture index lists 0 and 1for each field of a B frame, all the reference fields are combined inunits of a frame and then a reference frame is sorted in the same manneras the method of determining reference frame index of a B frame.Thereafter, the indexes that are increased by one are alternatelyallocated in an order that starts from a reference field having parityequal to a current picture to a reference field having parity differentfrom the current picture, while visiting the sorted reference frames insequence.

FIG. 4 shows reference picture indexes of top and bottom fields of a Pframe when a size of a reference buffer is 5, and FIG. 5 shows referencepicture index lists 0 and 1 of top and bottom fields of a B frame thatcan be used as a reference.

In order to improve coding efficiency, the moving picture coding systemusing multiple reference pictures provides a function of reordering thereference picture indexes before decoding a picture or a slice. Thereference picture indexes are arbitrarily reordered so as to enhance thecoding efficiency after determining an initial reference picture indexfor the frame and field pictures. FIG. 6 shows a result of determiningan initial reference picture index for a P frame and arbitrarilyreordering indexes according to the above-described method.

When the moving picture coding system using the multiple referencepictures codes an interlaced moving picture into a frame picture havingframe/field macroblocks, all the reference pictures for the framemacroblock are considered as a frame unit and one of the reference framepictures must be used for motion compensation. In addition, all thereference pictures for the field macroblock are considered asindependent pictures and one of the reference field pictures must beused for motion compensation.

Accordingly, in the frame picture coding that selects the frame/fieldcoding modes at the macroblock level, it is necessary to allocate valuesto the reference picture index in frame unit or field unit according tothe coding mode of each macroblock.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides reference pictureinformation used for motion compensation by determining the referencepicture index differently according a coding mode of macroblock wheninterlaced moving picture is coded with a frame picture havingframe/field macroblocks in a moving picture coding system using multiplereference pictures.

In one embodiment, the method of selecting a reference picture forprocessing a field macroblock includes selecting at least one referencepicture for processing a field macroblock from at least one referencepicture list using reference picture index information. Here, eachselected at least one reference picture is a field. For example, thereference field may a top field or a bottom field.

In another embodiment, the method may further include processing thefield macroblock using the selected at least one reference picture.

In a further embodiment, the method may include forming the at least onereference picture list. For example, the forming step may includeinitializing the at least one reference picture list, and reordering theat least one reference picture list.

In another embodiment, if the reference picture index information is aneven index number, then the selected at least one reference field has asame field parity as the field macroblock.

In yet another embodiment, if the reference picture index information isan odd index number, then the selected at least one reference field hasa field parity different from the field macroblock.

In one embodiment, the method of selecting a reference picture forprocessing a field macroblock includes selecting at least one referencepicture for processing a field macroblock from at least one referencepicture list using reference picture index information. Here, eachselected at least one reference picture is a field, and a parity of theselected at least one reference field may be based on the parity of thefield macroblock and the reference picture index information.

In one embodiment, the method includes obtaining a reference picturelist including multiple reference pictures, and obtaining a fieldreference picture index associated with the current field macroblock. Aframe reference picture index is determined based on the field referencepicture index divided by 2, and a field reference picture is selectedbased on the frame reference picture index and the field referencepicture index.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

In the drawings:

FIG. 1 shows macroblock pair in frame picture having general frame/fieldmacroblock;

FIG. 2 shows reference picture indexes of P frame coding in a movingpicture coding system according to the related art;

FIG. 3 shows the reference picture index lists 0 and 1 of B frame codingin a moving picture coding system according to the related art;

FIG. 4 shows reference picture indexes in a coding of top and bottomfields of a P frame in a moving picture coding system according to therelated art;

FIG. 5 shows reference picture index lists 0 and 1 in a coding of topand bottom fields of a B frame in a moving picture coding systemaccording to the related art;

FIG. 6 shows reference picture indexes reordered in a coding of a Pframe in a moving picture coding system according to the related art;

FIG. 7 shows reference picture indexes of a P frame and a B frame havingframe macroblocks in a moving picture coding system according to thepresent invention;

FIG. 8 shows reference picture indexes of P frame and B frame havingfield macroblock in a moving picture coding system according to anembodiment of the present invention (case 1);

FIG. 9 shows reference picture indexes of a P frame and a B frame havingfield macroblocks in a moving picture coding system according to anotherembodiment of the present invention (case 2);

FIG. 10 shows reference picture indexes of a P frame having fieldmacroblocks in a moving picture coding system according to anotherembodiment of the present invention (case 3);

FIG. 11 shows reference picture indexes of a B frame having fieldmacroblocks in a moving picture coding system according to a furtherembodiment of the present invention (case 3); and

FIG. 12 shows reference picture indexes of a P frame and a B framehaving field macroblocks in a moving picture coding system according toanother embodiment of the present invention (case 4).

DETAIL DESCRIPTION OF EXAMPLE EMBODIMENTS

Hereinafter, example embodiments of the present invention will bedescribed in detail with reference to accompanying drawings.

As stated previously, moving picture coding systems compensate formotion using motion vector information. In the case of a moving picturecoding system that uses multiple reference pictures, reference pictureindex information as well as motion vector information is used formotion compensation. The reference picture index is a value that is usedto distinguish multiple reference pictures from each other. A codertransfers the reference picture index to a decoder. The decoder selectsthe reference picture as indicated by the reference picture index, andperforms motion compensation using the selected reference picture.

When an interlaced moving picture is coded into a frame picture havingframe/field macroblocks, a moving picture coding system using multiplereference pictures performs a motion compensation from a reference framein order for frame coding in the frame macroblock. In other words, thereference pictures may be considered as a frame unit. The moving picturecoding system, however, performs a motion compensation from a referencefield in order for a field coding in the field macroblocks. In otherwords, the reference pictures may be considered as a field unit.

Accordingly, the frame pictures having frame/field macroblock useindexes having different structures at the macroblock level. Forexample, the frame macroblock uses a reference picture index of a frameunit and the field macroblock uses a reference picture index of a fieldunit. It should be considered that the reference picture indexes may bereordered at a picture or slice level.

Therefore, the frame picture having frame/field macroblocks shouldconsider the reference picture in a frame unit at a picture or slicelevel and have the reordered reference picture indexes of a frame unit.The reference picture index may be determined according to a coding modeof macroblock based on the reference picture indexes of the frame unitat the macroblock level.

The present invention provides an embodiment of a method for determiningreference picture indexes in a frame picture having frame/fieldmacroblocks. The method includes a process of determining referencepicture indexes at a picture (or slice) level and a process ofdetermining reference picture indexes at a macroblock level.

[1] Determination (or Initialization) of Reference Picture Indexes at aPicture (or Slice) Level

The reference pictures may be considered in a frame unit at a picture orslice level and reference picture indexes of frame unit are calculated.

In the method of determining a reference picture index for a P frame,reference frames are sorted in an order reverse to a coding order andindexes are allocated while visiting the sorted reference frames insequence.

Reference picture indexes for a B frame are determined based on adisplay order of the reference frames. First, in case of a referenceframe list 0, indexes are allocated in a reverse order to referenceframes whose display orders lag behind the B frame and the remainingindexes are allocated in the display order to reference frames whosedisplay order lead the B frame.

In case of a reference frame list 1, in contrast to the reference framelist 0, indexes are allocated in the display order to reference frameswhose display orders are higher than the B frame and the remainingindexes are allocated in a reverse order to reference frames whosedisplay orders are lower than the B frame.

In this and the other embodiment described below, in order to improvecoding efficiency, the moving picture coding system using multiplereference pictures may provide a function of reordering the referencepicture indexes before decoding a picture or a slice. The referencepicture indexes are arbitrarily reordered so as to enhance the codingefficiency after determining an initial reference picture index for theframe and field pictures. When the reference picture indexes arereordered so as to enhance a coding efficiency, the reference pictureindexes of a frame unit may be reordered.

[2] Determination (or Reordering) of Reference Picture Index atMacroblock Level

The reference picture index is modified according to the coding mode ofthe macroblock while performing a coding in unit of a macroblock pairwith respect to the reference picture indexes of a frame unit, which areobtained at the picture (or slice) level. This process will be describedbelow.

[2.1] Case of Frame Macroblock

Since motion compensation may be performed to frame macroblocks from areference frame, the reference picture index may be a frame unit.Accordingly, the reference picture indexes of a frame unit, which areobtained at the picture (or slice) level, are used.

A reference frame stored in a reference buffer may be configured with afield pair having parities (e.g., top and bottom) opposite to eachother. FIG. 7 shows reference picture indexes for a frame macroblock ina P frame and a B frame when a size of the reference buffer is 5.

[2.2] Case of Field Macroblock

Since motion compensation may be performed to field macroblocks from areference field, the reference picture index may be a field unit. Inthis case, a field pair having parities opposite to each other is alsopresent in the reference buffer. Accordingly, each reference frame isdivided into two fields while sequentially visiting the reference frameaccording to the reference picture indexes of a frame unit, which areobtained at the picture (or slice) level. Then, the reference pictureindexes are newly allocated to each field. The reference picture indexesfor two fields of each reference frame are allocated in various methods(Cases 1 to 4).

(Case 1)

Regardless of the top and bottom field macroblocks of a current fieldmacroblock, a lower index is allocated to the top reference field and ahigher index is allocated to the bottom reference field whilesequentially visiting the reference frames in an order of the referencepicture indexes, which are obtained at the picture (or slice) level.

In other words, the top and bottom reference field indexes may be givenby the equation below.Top reference field index=2×picture index of reference frameBottom reference field index=2×picture index of reference frame+1

FIG. 8 shows reference picture indexes for field macroblocks of a Pframe and a B frame.

(Case 2)

Regardless of the top and bottom field macroblocks of a current fieldmacroblock, a lower index is allocated to the bottom reference field anda higher index is allocated to the top reference field whilesequentially visiting the reference frames in an order of the referencepicture indexes, which are obtained at the picture (or slice) level.

In other words, the top and bottom reference field indexes may be givenby an equation below.Top reference field index=2×picture index of reference frame+1Bottom reference field index=2×picture index of reference frame

FIG. 9 shows reference picture indexes for field macroblocks of a Pframe and a B frame.

(Case 3)

Indexes that are increased by one are alternately allocated to referencefields, starting from the reference field having a parity equal to thecurrent field to the reference field having a parity different from thecurrent field, while sequentially visiting the reference framesaccording to a reference picture index order of the picture (or slice)level. FIGS. 10 and 11 show reference picture indexes for the fieldmacroblocks of a P frame and a B frame.

(Case 4)

Regardless of the top and bottom field macroblocks of a current fieldmacroblock, a lower index is allocated to reference field close to thecurrent field in view of time and a higher index is allocated toreference field farther from the current field in view of time whilesequentially visiting the reference frames in an order of the referencepicture indexes of the picture (or slice) level.

In other words, the reference field indexes may be given by the equationbelow.Reference field index close to the current field=2×picture index ofreference frameReference field index far from the current field=2×picture index ofreference frame+1

In FIG. 12, there are shown reference picture indexes for a fieldmacroblock of a P frame and a B frame.

While the present invention has been described and illustrated hereinwith reference to the example embodiments thereof, it will be apparentto those skilled in the art that various modifications and variationscan be made therein without departing from the spirit and scope of theinvention. Thus, it is intended that the present invention covers themodifications and variations.

According to a coding a moving picture method of at least one embodimentof the present invention, when a moving picture coding system usingmultiple reference pictures codes an interlaced moving picture into aframe picture having frame/field macroblocks, the moving picture codingsystem efficiently provides reference picture information used formotion compensation by differently determining reference picture indexesaccording to coding modes of macroblocks.

1. A method of decoding a current field block by selecting a referencefield picture performed by a moving picture decoder, comprising:obtaining, by the moving picture decoder, a reference frame picture listof multiple reference frame pictures; determining, by the moving picturedecoder, a reference frame picture index based on display orderinformation for the multiple reference frame pictures; obtaining, by themoving picture decoder, a reference field picture index associated withthe current field block; determining, by the moving picture decoder, areference frame picture from the reference frame picture list based onthe reference field picture index divided by 2; selecting, by the movingpicture decoder, a reference field picture for the current field blockbased on the obtained reference field picture index and the determinedreference frame picture, wherein the selecting step selects a referencefield picture having a same field parity as the current field block ifthe obtained reference field index is an even index number, otherwisethe selecting step selects a reference field picture having a differentfield parity from the current field block; and decoding, by the movingpicture decoder, the current field block by referring to the selectedreference field picture.